This invention relates to a method and an apparatus or the preparation of clean gases to be brought into contact with substrate surfaces. More specifically, the invention relates to the art of preparation of clean gases for preventing the contamination of the surfaces of substrates as raw materials, semi-finished products and finished products in high-technology industries such as the manufacture of semiconductors and liquid crystals.
The scope of application of the present invention covers, for example: (1) contamination control of wafers in the process of semiconductor manufacture; (2) contamination control of glass substrates in the process of liquid-crystal manufacture; and (3) contamination control of substrates in the process of precision machinery manufacture. Exemplary areas where the method and apparatus of the invention for preparing clean gases can be applied are spaces within cleanrooms in such manufacturing shops as those concerned with semiconductors, liquid crystals and precision machines, as illustrated by safety cabinets, clean boxes, valuables storage spaces, wafer storage spaces, sealed transport spaces for valuables, clean sealed spaces and transport spaces in the presence of various gases or under reduced pressure or in vacuo, spaces containing gases to be supplied to cleaning equipment, and spaces containing air to be supplied to create air knives.
The prior art technology will now be described with reference to the case of cleaning air in cleanrooms at semiconductor manufacturing shops.
In cleanrooms, fine particles (particulate matter) and gaseous substances such as extremely low concentrations of non-methane hydrocarbons (HCs) in air originating typically from automotive emission gases are of importance as contaminants. In particular, HCs, if present at extremely low concentrations in ordinary air (both inside and outside rooms), will cause contamination as gaseous deleterious components and, hence, need be removed. Various solvents (e.g. alcohols and ketones) that accompany operations performed in cleanrooms are also of importance as contaminants.
Stated more specifically, if the above-mentioned contaminants (fine particulate and gaseous contaminants) deposit on the surfaces of substrates as wafers, semifinished products and finished products, the substrate surfaces are prone to be damaged and this can be a cause of a lower production rate (yield) of semiconductor products; hence, those contaminants have to be removed. Both fine particles and gaseous substances will increase the angle of contact on substrate surfaces and, particularly in cleanrooms, HCs have been found to have a great tendency to increase the angle of contact. The term xe2x80x9cangle of contactxe2x80x9d as used herein refers to the angle of contact due to wetting by water and indicates the degree of contamination on substrate surfaces. Stated more specifically, when a hydrophobic (oily) substance deposits on a substrate surface, said surface repels water to become less likely to be wetted. Then, the angle of contact between the substrate surface and the drop of water will increase. Therefore, a large angle of contact means a high degree of contamination and, conversely, a small angle of contact means a low degree of contamination.
Conventional methods of cleaning air in cleanrooms or apparatus therefor are roughly classified in two categories:
(1) a mechanical filtering method (e.g. a HEPA filter); and
(2) a filtering approach (e.g. HESA filter) that entraps fine particles electrostatically by means of a high-voltage charged or conductive filter. Either of these methods; aims at removing fine particles and is ineffective for removing gaseous contaminants such as non-methane hydrocarbons (HCs) that will increase the angle of contact.
On the other hand, HCs which are gaseous contaminants are known to be removable by such methods as combustion decomposition and O3 decomposition. However, these methods are ineffective for removing extremely low concentrations of HCs present in air to be introduced into cleanrooms.
Gaseous deleterious components other than HCs include SOx, NOx, HCl, NH3, etc. and these are known to be removable by relying either upon neutralization reactions using appropriate alkaline or acidic substances or upon oxidation reactions. However, these methods are also less effective if the concentrations of the components of interest are extremely low as in the case where they are present in air to be introduced into cleanrooms.
The present inventors already proposed methods and apparatus for preventing the contamination of substrate surfaces by using adsorbents or absorbers with a view to preventing the increase in the above-defined angle of contact (Japanese Patent Application Nos. 91/341802 and 92/180538). These methods and apparatus are effective in certain areas of application but further improvements are necessary in order to increase their practical feasibility.
Stated more specifically, particulate matter and gaseous deleterious components that increase the angle of contact must be removed in order to improve the production rate of semiconductor products. Hence, the object of the present invention is to provide a method and an apparatus for preparing clean gases in which the concentrations of fine particles and gaseous deleterious that will increase the angle of contact on substrate surfaces are reasonably low.
To attain this object, the present invention provides a method for preparing a clean gas that is to be brought into contact with the surface of a substrate for preventing it against contamination, which method comprises reducing the concentration of water in a gas to be treated to 50% (RH) or below by dehumidifying means and thereafter cleaning the gas by dust control means and adsorption and/or absorption means so that the concentration of fine particles in the gas is reduced to class 10 or below whereas the concentrations of non-methane hydrocarbons are reduced to 0.2 ppm or below.
The invention further provides an apparatus for preparing a clean gas that is to be brought into contact with the surface of a substrate for preventing it against contamination, which apparatus has dehumidifying means at the entrance thereof for reducing the concentration of water in a gas to be treated to 50% (RH) or below, as well as dust control means and adsorption and/or absorption means that are located downstream of said dehumidifying means so that the concentration of fine particles in the gas which has been reduced in water concentration to 50% or below is reduced to class 10 or below by said dust control means whereas the concentrations of non-methane hydrocarbons are reduced to 0.2 ppm or below by said adsorption and/or absorption means.